1. Field of the Invention
This invention pertains to the arrangement of gears, shafts, and synchronizers of an automotive transmission, especially to such a transmission of the type having a countershaft or layshaft disposed parallel to a power input shaft.
2. Description of the Prior Art
When the engine of a motor vehicle is idling and the neutral clutch that connects the engine to a manual transmission is disengaged, the speed of the engine varies as a result of cyclic irregularity intrinsic in the operation of an internal combustion engine. Countershaft manual transmissions for automotive use contain gear sets whose members are in continuous meshing engagement, though the gear selector of the transmission is in the neutral position. When the maindrive clutch is engaged and the gear selector is in the neutral position, speed changes of the engine transmitted to the gearset produce noise called "neutral rollover gear rattle." When the engaged gear teeth change position with respect to each other due to engine speed variations, the teeth of the driven gear contact alternately the driving face and the nondriving face of the driving pinion.
To reduce or eliminate this objectionable noise, various devices have been employed to take up clearances between the faces of the gear and mating pinion. These devices include a wheel coaxially arranged with the gear having similar teeth to those of the gear, and means for elastically maintaining the teeth of the gear in position that is angularly offset relative to the gear. A mechanical spring, usually a helical spring, located radially offset from the axis of the gear, applies a force directed tangentially to the axis tending to rotate the angular element with respect to the gear wheel. Other means such as elastomeric material have been used in the prior art to bias an angular member angularly with respect to a gear to maintain contact between the gear teeth in order to avoid gear rattle.
U.S. Pat. No. 4,718,295 describes use of a one-way clutch located on the input shaft between a gear wheel and a cylindrical surface of the shaft. The one-way clutch permits a one-way drive connection so that power can be transmitted through the clutch from the input shaft to either of two countershafts when fourth and fifth gear ratios are selected by the vehicle operator.
It is an object in the design of a manual transmission to minimize the effort required to make gear ratio changes, i.e., to reduce as much as possible the magnitude of force that must be applied to a shift mechanism to disengage a current gear ratio and to engage a new gear ratio. The magnitude of that force is to a great extent determined by the magnitude of inertia of rotating components that is reflected to the synchronizer units. Minimizing the weight of components of a motor vehicle is desired in order to reduce fuel consumption. Various attempts have been made to reduce noise produced by a manual transmission and to reduce rotating inertia of the gears, shafts, synchronizers and other such components.